Figure 2: Experimental setup.

(a) Schematic of the experimental setup. RF voltage pulses are generated from a fast photodiode illuminated by the femtosecond (fs) laser beam (with a centre wavelength of 820 nm). The RF pulses are amplified by a power RF amplifier. A bias tee adds a DC offset to the RF pulses. THz pulses are generated by illuminating a biased interdigitiated antenna with a femtosecond pump beam. The THz pulses are coupled into a facet of the quantum cascade laser (QCL) with parabolic mirrors. The QCL output field from the other facet is measured using electro-optic sampling in a ZnTe crystal with a femtosecond sampling beam. (b) Time-averaged light-current density (L-J) curve of the QCL with RF OFF (black) and RF ON (purple). The QCL output power is measured with a slow pyroelectric detector that averages out any power variations caused by the RF pulses. (c) Instantaneous voltage of RF pulses, measured with a 1 GHz oscilloscope and 20 dB attenuation. The two peaks correspond to two consecutive laser pulses. The negative voltages between RF pulses arise because the RF pulses are alternate current (AC) coupled and the time-averaged RF voltage must therefore be zero.